In wireless communication systems, an input signal is provided to a transmitter for transmission therefrom. Typically prior to transmitting the signal, the transmitter conditions the signal so that it is in a form suitable for transmission. Such conditioning typically involves power amplification, where it is desirable to operate power amplifiers (PAs) near saturation in order to attain high power efficiency. Unfortunately, such operation typically introduces non-linear distortion in the amplified signal. This is evidenced by spectrum re-growth, which diverts some of the energy from a desired frequency channel into adjacent frequency channels. This results in a loss of performance within a desired frequency channel as well as the creation of interference within adjacent frequency channels.
For space applications, analog linearization circuits are often used in conjunction with PAs and are manually tuned for each PA in an attempt to provide optimum performance. In general, this is costly but considered necessary for space applications where total available power for signal transmission is very limited. For ground terminal use, the use of a linearization circuit is typically not implemented due to its cost. Instead, a more powerful PA is utilized and operated in an input back-off mode of operation—with a reduced output power level. The reduction in power amplifier output capability is typically a less costly implementation than the use of a linearization circuit. However, at microwave and mm-wave frequencies above around 30 GHz the output power capability of PA's is currently very limited and thus a more efficient use of the available power would significantly reduce the cost. In some cases, about 70% of the cost of 30/20 GHz satellite terminal is in the RF front end, with approximately half of this cost associated with the PA.
Furthermore, above 1 W the cost of an SSPA today increases linearly with increased output power. Given that 1-2 dB back-off is usually required for QPSK signals, which results in a loss of 20-40% efficiency, any increase in efficiency is likely to translate to a significant power advantage especially with the advent of 8-PSK and 16-QAM in satellite systems.
In the prior art, there are numerous techniques that have been employed in order to operate power amplifiers near saturation to attain high power efficiency while maintaining amplified signal linearity.
The prior art is limited in that spectrum efficient modulation, such as a multi-level quadrature amplitude modulation, or pulse shaping filters with a small roll-off factor, resulting in a significant envelope variation, often cannot be used due to the need to operate the PA within its linear region. Thus, efficiency of the PA is reduced. Additional limitations found in the prior art are a need to down-convert and demodulate the PA output signal, the comparing of the input signal to the reconstructed signal from the output port of the PA, a need to interrupt signal transmission, or they are limited to non-transmit periods of time, which is suitable for TDMA systems but not to FDMA systems. Additionally, a need for a training sequence and in some cases an accurately generated reference signal, or high computational requirements resulting in convergence to a solution being non trivial. Lastly, many of the prior art techniques require sampling of the PA output signal at a high sampling rate that is higher than the symbol rate of the input signal—most often at or above the Nyquist rate.